Desensitization of LH secretion by the gonadotrope is a hallmark of GnRH action unless it is administered or secreted in a pulsatile fashion. The cellular and molecular mechanisms of gonadotrope desensitization is poorly understood. A class of proteins discovered originally in yeast have been described as being involved in mammalian cell desensitization. Called regulators of G-protein signalling (RGS), one of these proteins (RGS-3) has been shown by us to inhibit GnRH-stimulated cellular inositol trisphosphate (IP3) levels, its mRNA is present in the gonadotropic aT3-1 cell line, and RGS is detected by antibodies in membranes of aT3-1 and pituitary cells. RGS3 is shown to bind the Gqa protein in vitro supporting the notion that RGSs act by binding G proteins, thus inhibiting G-protein coupling of the receptor to phospholipase C activation. Fifteen mammalian genes encoding RGSs have been identified. The Specific Aims are to: (1) complete the preliminary characterization of RGS3 as participating in GnRH-induced desensitization; (2) Determine if other members of the 15 member RGS family are involved in GnRH-induced desensitization by screening RNA from GnRH-treated aT3-1 cells; (3) Investigate the role of RGS in GnRH-induced desensitization of LH secretion in purified rat pituitary gonadotropes. The experiments have the potential to provide a cellular and molecular explanation of GnRH-induced desensitization, and perhaps provide an entree to the wider issue of regulation of general gonadotrope responsiveness.